The near camera range apparently has far more affect on z-buffering usage than the far camera range. As I understand it, the z-buffer works logarythmically, with the detail density increasing dramatically the nearer the polygons are to the camera.
Here's a link which explains it better.
http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html
Here's a link which explains it better.
http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html